1 Field of the Invention
The present invention relates to a device and to a method for reading a capacitive sensor, in particular of a micro-electromechanical type.
2 Description of the Related Art
As is known, the use of capacitive sensors is continuously spreading to numerous applications, in which the reduction of consumption is a fundamental target. For example, capacitive inertial micro-electromechanical-system (MEMS) sensors of a differential type are increasingly frequently used in a wide range of portable electronic devices, such as cell phones, palm-top computers, digital camcorders and cameras, and the like, which are supplied autonomously by batteries. Clearly, in cases of this sort the reduction of the consumption is indispensable for increasing the autonomy of the device.
In order to minimize the power absorption, very frequently traditional continuous-time read circuits for capacitive sensors have been replaced by switched-capacitor (SC) read circuits, which are much more suitable for operating with low supply voltages and an extremely low current consumption. In a parallel manner, reading techniques have been developed for optimizing the reading precision and sensitivity. For example, the so-called “correlated-double-sampling” (CDS) technique enables effective elimination of the disturbance caused by possible offsets and low-frequency noise (1/f noise, or flicker noise) of the electronics used (typically, a charge-voltage converter including a charge amplifier).
By way of example, FIGS. 1-3 show the different steps for reading a capacitive inertial sensor 1 of a differential MEMS type, using the CDS technique. In particular, in FIGS. 1-3 the inertial sensor 1 is represented by means of an equivalent electrical diagram and comprises a first sense capacitor 2a and a second sense capacitor 2b having a first common terminal, which forms a driving terminal 1c of the inertial sensor 1. Second terminals of the first sense capacitor 2a and of the second sense capacitor 2b form a first sense terminal 1a and, respectively, a second sense terminal 1b of the inertial sensor 1. In practice, the two capacitors 2a, 2b have differentially variable capacitances, i.e., they have the same capacitance at rest CS, when the inertial sensor 1 is not subjected to the quantity to be sensed, and show capacitance variations of equal amplitude and opposite sign when the inertial sensor 1 senses a quantity along a pre-set axis.
A read circuit 3 is associated to the inertial sensor 1 and comprises a signal source 4, a charge-voltage converter 5, and a canceling stage 7.
The signal source 4 is connected to the driving terminal 1c of the inertial sensor 1 and supplies a step read voltage VRD.
The charge-voltage converter 5 includes a fully differential switched-capacitor charge amplifier 10, having a first integration capacitor 11a connected between a first input and a first output and a second integration capacitor 11b connected between a second input and a second output. Furthermore, the first input and the second input of the charge amplifier 10 are connected to the first sense terminal 1a and to the second sense terminal 1b of the inertial sensor 1, respectively.
The canceling stage 7 comprises a first hold capacitor 12a and a second hold capacitor 12b, respectively connected in series to the first output and to the second output of the charge amplifier 10. Furthermore, terminals of the first hold capacitor 12a and of the second hold capacitor 12b form a first output 3a and, respectively, a second output 3b of the read circuit 3.
In a first step, or reset step, the signal source 4 (herein illustrated with a dashed line) sends the driving terminal 1 c of the inertial sensor 1 to a ground value. The first input and the second input of the charge amplifier 10 are instead brought to a reference line 15, which supplies a constant reference voltage VREF, whereas the first output and the second output are short-circuited. For this purpose, first reset switches 16a, 16b, connected between the reference line 15 and a respective input of the charge amplifier 10, and a second reset switch 16c, arranged between the outputs of the charge amplifier 10 itself, go into a closed condition.
In a second step, or offset-canceling step, the first reset switches 16a, 16b and the second reset switch 16c are opened condition, while the driving terminal 1c of the inertial sensor 1 is still kept at the ground voltage. Furthermore, a first canceling switch 18a, connected between the reference line 15 and the first output 3a, and a second canceling switch 18b, connected between the reference line 15 and the second output 3b, are closed. In this way, any possible disturbance, such as 1/f noise, and a possible offset introduced by the charge amplifier 10 cause a canceling voltage VC between the outputs by the charge amplifier 10 itself. The canceling voltage VC is in practice stored in the first hold capacitor 12a and in the second hold capacitor 12b. 
Finally, a third step or sensing step is executed, in which the first reset switches 16a, 16b, the second reset switch 16c, and the canceling switches 18a, 18b are opened, and the signal source (illustrated with a solid line) supplies the step read voltage VRD to the driving terminal 1c of the inertial sensor 1. Through the first sense capacitor 2a and the second sense capacitor 2b, a first sense charge QA and, respectively, a second sense charge QB, correlated to the capacitive unbalancing of the inertial sensor 1, are supplied to the inputs of the charge amplifier 10 and converted into a rough output voltage VOR, which includes the contributions of noise and of offset due to the charge amplifier 10. The canceling stage 7 subtracts the canceling voltage VC, stored by the first hold capacitor 12a and the second hold capacitor 12b. In response to the step of the read voltage VRD, then, the read circuit 3 supplies a corrected output voltage VOC, which substantially is not affected by the contribution of the low-frequency noise and of the offsets introduced by the electronics.
Albeit effective for eliminating the disturbance described, the CDS technique does not, however, enable suppression of the low-frequency disturbance generated upstream of the charge amplifier 10. Said disturbance can have different origins, but, basically, takes the form of a differential disturbance current ID supplied in parallel to the currents due to the injection of charge (QA, QB) performed by the inertial sensor 1 (the disturbance is represented schematically by a current generator 20 in FIG. 4). In particular, an important source of disturbance is represented by the first reset switches, which, even when opened, have dispersion currents (of an intensity of up to a few nanoamps). The problem, in addition, becomes increasingly more important because the need to reduce also the overall dimensions pushes in the direction of using as switches MOS transistors with very short channel, which suffer more from current leakages. Other causes of disturbance are the inevitable thermal drifts and the ageing of the components, so that spurious resistive paths may be formed within the inertial sensor 1.